Been running in the cold swapping between a couple garments for testing. Straight Merino wool and the Patagonia R1 hoody. Both are perfect for events like this. The wool is a bit better at blocking wind, but you give up some breathability and you will get wet underneath. The good thing is, wool still warms when wet. The R1 Grid fleece will keep you nearly bone dry, but it will not block wind at all. If there is a nippy wind, I would run in the Merino. If the humidity is high and not much of a cold breeze, I would run in the R1 or some other similar grid fleece. Either would work, and if super chilly, you could wear the wool under the R1. You could run well below freezing with just those two layers and be comfortable.